The invention relates to devices for processing images of a portion or portions of a body, and more precisely devices for automatically registering pairs of two-dimensional (2D) or three-dimensional (3D) images of the same portion of a body, obtained by magnetic resonance and by ultrasound, respectively.
The invention relates to devices for processing images of a portion or portions of a body, and more precisely devices for automatically registering pairs of two-dimensional (2D) or three-dimensional (3D) images of the same portion of a body, obtained by magnetic resonance and by ultrasound, respectively.
By a “portion of a body” is meant here any part of a human or animal body consisting of tissues and/or organs that can be observed at least partially by means of magnetic resonance techniques and ultrasound techniques (B-mode echography).
In some fields, such as medicine, for example, magnetic resonance images or ultrasound images are used to make diagnoses on portions of a body, or even to carry out assisted surgery. The more precise the surgical interventions have to be (taking into account the operating environment), the more reliable and precise the images used have to be. This is the case particularly, although not restrictively, in the field of neurosurgery on the brain.
As the two methods of acquiring images mentioned above each supply specific data (reflection level for ultrasound and water concentration for magnetic resonance), it is therefore important to try to register with one another (or match) the images that they supply so that they can be superimposed precisely and thus provide reliable and complementary data for certain structures of interest.
Unfortunately, as those skilled in the art will know, the registration of such images is made particularly difficult by the fact that the two techniques for image acquisition supply information that is represented in very different ways (on account of the physics of image formation) and produce artefacts that are different in nature and origin (such as for example acoustic shadows (obscuring part of the signal) or speckle noise). Moreover, this registration rapidly becomes unstable.
Several conventional registration methods have been proposed, but none of them is truly effective and/or useable in real time, particularly in the field of image-assisted neurosurgical operations. This is the case particularly with the techniques known as “sum squared difference”, “mutual information” and correlation ratio”.
It has also been proposed to improve some of the above-mentioned registration methods either by carrying out prior registration of features of homologous images extracted from the images, or by pre-treating the images so as to render them more similar and thus make it possible to use conventional measurements of similarity, or by carrying out so-called “iconic” registration based on a specific measure of similarity intended to tune the intensities of the images.
The pre-treatment of the images is based on the definition of regions of interest in the images by magnetic resonance and ultrasound, using dots, lines and/or areas. The main drawback of this pre-treatment is the fact that it renders the registration very sensitive to the (manual) segmentation of the images.
Iconic registration is based on the use of measurements of similarity based on image intensity. It eliminates the main drawback of image pre-treatment but involves searching for measurements of similarity starting from statistical measurements on histograms, which may lack specificity and result in a failure of registration.
As there is no satisfactory known technique for registering images by magnetic resonance and ultrasound, the invention therefore sets out to improve the situation.